The fuel tanks at Gaza’s only power plant came under attack early Tuesday, threatening to deepen an already dire humanitarian situation.

The attack came hours after Israel’s Prime Minister Benjamin Netanyahu warned in a televised speech of a "prolonged" campaign in Gaza against Hamas. Israel carried out more than 70 strikes overnight - one of the biggest bombardments in the nearly month-long campaign. A Palestinian health official told The Associated Press that at least 100 Palestinians were killed in Israeli airstrikes and tank shelling on Tuesday.

Thick black smoke billowed over Gaza City as the fire raged at the power plant, which is the enclave's second-largest source of electricity and the only provider inside the territory for its 1.8 million citizens.

Fathy Sheikh Halil, who is the director of the Gaza Electricity Station Company, said the damage would take months to repair.

Jamal Dersawi, the plant’s general manager, confirmed that the major fuel storage tanks supplying the facility took a hit before dawn. There was no immediate comment from Israel’s military about the attack.

Dersawi said many of the plant's employees were fearful of remaining at the site due to the potential for further attacks.

Gaza gets most of its electricity via power lines from Israel. However, several of those have been damaged since the war between Israel and the Palestinian militants began more than three weeks ago. The plant provides about one-third of Gaza's power.

It also came under attack in June 2006, when Israeli aircraft knocked out its transformers.

Dersawi said that since the facility provides power to key infrastructure, including water sanitation facilities, Tuesday’s attack will have a devastating impact on the citizens of Gaza.

“It will affect the 1.8 million people from Rafah to Beit Hanoun,” he said.

Ismail Haniyeh, the Hamas leader whose house was struck in an airstrike early Tuesday, said in a statement that "destroying stones will not break our determination," according to The Associated Press.

Despite intense international efforts and mounting pressure, a lasting ceasefire has failed to materialize. The three weeks of fighting between Israel and Hamas has left more than 1,100 dead in Gaza, according to Gaza’s Health Ministry, mostly civilians. On the Israeli side, 53 soldiers have died, along with two Israeli civilians.

NBC News' Wajjeh Abu Zarifa and James Novogrod and The Associated Press contributed to this report.